The present invention relates generally to illumination optical systems, and more particularly to an illumination apparatus in an illumination apparatus and an exposure apparatus used to expose objects such as single crystal plates for semiconductor wafers, glass plates for liquid crystal displays (LCD), and the like. The present invention is suitably applicable, for example, to an illumination apparatus used for an exposure apparatus that exposes single crystal plates for semiconductor wafers in a step-and-scan projection manner in a photolithography process.
Along with the recent demands on smaller and lower profile electronic devices, minute semiconductor devices to be mounted onto these electronic devices have been increasingly required. For example, a design rule for a mask pattern requires that an image with a size of a line and space (LandS) of 0.1 xcexcm or less be extensively formed and, presumably, it will further move to a formation of circuit patterns of 80 nm or less in the future. LandS denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus, which is a typical exposure apparatus for fabricating semiconductor devices, generally includes an illumination optical system that illuminates a mask, and a projection optical system that is located between the mask and an object to be exposed. In order to obtain a uniformly illuminated area, the illumination optical system introduces a beam from a light source to an optical integrator in order to use multiple light sources created at and around the optical integrator""s plane of exit as a secondary light source to illuminate a mask plane via a condenser lens.
The resolution R of the projection exposure apparatus is given using a wavelength of the light source (xcex) and the numerical aperture (NA) of the projection optical system in the following Rayleigh""s equation:
R=k1(xcex/NA)xe2x80x83xe2x80x83(1)
where k1 is a constant determined by a development process and others, and approximately 0.5xcx9c0.7 in a normal exposure.
As mentioned above, the recent demands for highly integrated devices have increasingly required fine patterns to be transferred, i.e., high resolution. It is conceivable from the above equation that a higher numerical aperture NA and reduced wavelength xcex would be effective for higher resolution. In recent years, a wavelength of an exposure light source is shifting from KrF excimer laser (with a wavelength of 248 nm) to ArF excimer laser (with a wavelength of 193 nm), and NA is from about 0.6 to about 0.75. Further, a practical application of F2 excimer laser (with a wavelength of 157 nm) is being promoted as an exposure light source.
In the meantime, a design of a lens becomes difficult as a numerical aperture increases, while aberration relates to the numerical apertures. For example, a spherical aberration is proportional to the 3rd order of an NA according to the 3rd order aberration theory. Thus, as the NA increases, the design of a lens becomes difficult. In addition to the numerical aperture, an angle of view (an area that may be projected by a projection lens) also relates to the aberration. For example, a distortion is proportional to the 3rd order of an angle of view according to the 3rd order aberration theory. Therefore, as the angle of view increases, the design of a lens also becomes difficult.
It is the recent trend in the lithography process in the semiconductor fabrication to use a scanning projection exposure apparatus of a step-and-scan type for exposure. A scanning projection exposure apparatus has an advantage in that it may provide not only provide the high resolution and the wide field size, but also the smaller angle of view than a cell projection exposure apparatus and thus the reduced aberrations. Such a scanning projection exposure apparatus requires a uniform illuminance distribution to scan and expose a wafer plane for a pattern of high resolution, and thus an illumination apparatus for illuminating a mask plane is required to form a uniformly illuminated area.
However, it has not yet been verified whether a conventional illumination apparatus illuminates an object plane, such as a mask plane, uniformly and effectively (or with a desired illuminance). In general, an illumination apparatus in a scanning projection exposure apparatus forms a rectangular illuminated area and use it to illuminate the target plane, but the conventional illumination apparatus has had lower illuminance on the periphery in a longitudinal direction of the illuminated area than that in its center, thus causing non-uniform illumination. The non-uniform illumination causes an insufficient pattern transfer to a resist, and cannot provide high quality semiconductor wafers, LCDs, thin-film magnetic heads, and the like. The uniform accumulated illuminance would be available at the illuminated plane using a blade for defining the illuminated area (e.g., a slit with a variable aperture shape, which is sometimes referred to as a field stop) at the illuminated plane (or a plane conjugate with the mask plane) to narrower the width of the slit around the center in the longitudinal direction of the illuminated area than that of its periphery. The more reduced the illuminance on the periphery in the longitudinal direction is, the smaller the slit""s center width should be: This would increase the light blocking part, greatly lowering the light utilization efficiency of light used to illuminate the target plane, and thus the throughput.
Accordingly, it is an exemplary object of the present invention to provide an illumination apparatus with high light utilization efficiency, which gives uniform illumination in the longitudinal direction of the illuminated area formed by the illumination apparatus, as well as an exposure apparatus using the same.
An illumination apparatus of one aspect of the present invention includes an optical integrator for illuminating a target plane by using light emitted from a light source, the illumination apparatus forming an approximately rectangular illuminated area using light emitted from the optical integrator that includes a first optical system for enlarging the light in a longitudinal direction of the illuminated area, and a second optical system for enlarging the light in a lateral direction of the illuminated area, wherein a light exit plane (surface) of the first optical system is provided closer to the target plane than that of the second optical system. Such an illumination apparatus arranges the light exit plane of the first optical system for enlarging the beams in the longitudinal direction of the illuminated area is located closest to the target plane. Therefore, the first optical system may have a back focus shorter than the conventional one, reducing distortion in the longitudinal direction. As a result, the illuminated area that this illumination apparatus forms has uniform illuminance, thus improving light utilization efficiency. Here, the first optical system""s light incidence plane (surface) and the second optical system""s light incidence plane (surface) may be located to be approximately conjugate with the target plane. This illumination apparatus may have rectangular or arc illuminated area on the target plane.
In this illumination apparatus, the first optical system may include a pair of cylindrical lens arrays whose generating line is parallel to the lateral direction, whereas the second optical system may include a pair of cylindrical lens arrays whose generating line is parallel to the longitudinal direction, and wherein a light beam incident upon the optical integrator enters in order from a third lens set, a first lens set, a fourth lens set, and a second lens set where the first and second lens sets are the pair of cylindrical lens arrays in the first optical system in order from a light incidence plane, and the third and fourth lens sets are the pair of cylindrical lens arrays in the second optical system are in order from the light incidence plane.
Here, a light incidence plane of the first optical system and a light incidence plane of the second optical system may be located to be approximately conjugate with the target plane. The first optical system may include in the longitudinal direction an optical element that extends in the lateral direction, whereas the second optical system may include in the lateral direction an optical element that extends in the longitudinal direction, and wherein the first optical system may be located closer to the target plane than the second optical system. Such an optical element is a convex lens array or a concave mirror array (or a cylindrical lens array or a cylindrical mirror array). The first optical system""s light incidence plane of incidence and the second optical system""s light incidence plane may be located approximately conjugate with the target plane.
Further, an exposure apparatus as another aspect of the present invention includes any one of the above-mentioned illumination apparatuses and an optical system that projects a pattern formed on a reticle or mask onto an object to be exposed. Such an exposure apparatus includes the above illumination apparatus, and exhibits the same operations, as well as realizing projection and exposure with high light utilization efficiency and satisfactory throughput.
A device fabricating method as still another aspect of the present invention includes the steps of exposing the object by using the above exposure apparatus, and performing a specified process for the exposed object. Claims for the device fabricating method that exhibits operations similar to those of the above exposure apparatus cover devices as their intermediate products and finished products. Moreover, such devices include, e.g., semiconductor chips such as LSIs and VLSIs, CCDs, LCDs, magnetic sensors, thin-film magnetic heads, etc.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.